Ouch... if the one you've got is from 1999 and it hasn't been updated since then... then you really need a new one

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Not really sure, why the ones you downloaded didn't work, but I can personally recommend Norton AntiVirus... use it myself, and it has successfully grabbed all incoming viras - I know it's only a trial version, but it should be able to be downloaded
here at the Norton site.
There was a free (and pretty good I remember hearing) one some time ago... but I'm not sure if it's still available... can't even remember the name neither

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EDIT: Just found another anti-virus application by searching for 'free anti-virus' on Google.com, but I don't know how good it is, but it seems like being quite ok...
http://www.grisoft.com/html/us_downl.htm#FREE.